Radio receiving system



Dec. 27, 1938. c. J. VAN LOON RADIO RECEIVING SYSTEM I Filed Sept. 26, 1956 \NVENTOR CAREL JA 2A LOON Mivw sYvf

ATTORNEY Patented Dec. 27, 1938 UNITED STATES PATENT QFFICE RADIO RECEIVING SYSTEM v Carel Jan van Loon, Eindh oven Netherlands, as-

signon to N. V. Philips Gloeilarnpen fabrieken,

Eindhoven, Netherland Netherlands 5, am raman c th Application September 26, 1936, Serial No. 102,685

' In Germany October go, 1935 2 Claims. (c1. gsq go X cos wt(1+m cos pt) is received simultaneously with a strong 'unmodulated signal Y cos (w+q) t, to being the angular frequency of the carrier wave of the weak signal, p the angular frequency of the low-frequency oscillation with which said signal is modulated and (w-l-Q) the angular frequency of the strong signal.

The voltage supplied to the detector amounts in this case to: I

V=X cos wt(1+'m cos pt) cos (w-I-(J) t= [X(1+m cos 1915) +1 cos qtl.cosw t-Y sin qt. sin wt Instead thereof may be written Assuming that the oscillations having an angu lar frequency equal to qip, and 2a are suppressed,

is ustomary, by means of a tone filter, the current supplied to the loudspeaker is directly proportional to X2 2 2 Y|:1+% 1+ +2m cos pt+%- cos 2pt)] or approximately to Y[1+% ;m cos pt] whereas it was to be expected that said current would be proportional to and Ysin qt X(1Im cos pt)l-Y cos qt R represents the enveloping curve of the voltage supplied to the detector; in the case of linear detection the curve of the output low-frequency voltage of the detector has the same course as the enveloping curve. The above expression for R may be rewritten:

arc tan i. e., Y, the chief carrier +Xm cos pt, the variation supplied by the signal} The modulation of the Weak signal is consequently suppressed in the proportion which is called the modulation suppression factor,

45 Since X Y, on developing the radical expression into a series and on neglecting the terms comprising 29 50 I and higher powers of HIM or from another point of view, the amplitude of variationis reduced from Xm cos pt) to i= (Xm cos pt).

The effect described only occurs if the detector a s lfi c e sli ht inertia o fo l w th fr quency difference q so that in fact the enveloping curve is reproduced. The time constant of the detector should consequently be small; If said According to the invention, an increased selectivity in'a radio receiving system is obtained by supplying the modulated oscillations received to two detectors having different modulation suppression factors and by oppositely connecting the low-frequency output voltages of the two detectors in such manner that in the case of simultaneous 7 reception of two signals of different intensity one of the signals is completely or almost completely suppressed. V

Theinvention will beexplained more fully with reference to the accompanying drawing which represents one embodiment thereof.

The high-frequency oscillations set up across an oscillatory circuit K are supplied via condensers C1 and C2, high-frequency choke coils S1 and S2 7 and condensers C3 and C4 to two diode detectors D1 and D2, which results in the production of a low-frequency voltage drop in resistances R1 and R2. The condensers C1 and C2 and the resistances R1 and R2 are so chosen that the two detectors have different time constants and therefore different modulation suppression factors. The low-frequency voltage drop across part of the resistances R1 and R2 is supplied to the control gridsof push-pull connected amplifying tubes B1 and B2. The tapping points on the resistances R1 and R2 are so chosen that the amplitude of the undesired signal on the control grids of the two amplifying tubes is the same. Only the desired signal is present in this case in the secondary winding S of the balancing transformer.

The adequate choice of resistances and condensers permits to make the'ratio between the amplitudes of the desired and the undesired signal in the output'voltage of one of the detectors four times as large as in the output voltage of the other detector. In this case of the amplitude of the desired signal is consequently maintained.

In order to further explam how Increased signal-modulated oscillations, comprising a tuna- -ble circuitga'pair of detectors having different modulation suppression factors connected across lectivity is obtained, by the suppression of the effect due to an interfering signal being simultaneously received with the desired signal, it will be assumed that two modulated signals A1 cos w 1t(l+m1 cos pit) In this equation 0:1 represents a constant, which is dependent upon the efficiency of the detector Drand of the setting of'the voltage divider R1, 7

whereas n represents the demodulation factor.

To the control grid of the valve B2 is supplied an audio frequency voltage:

The current flowing through the primary of the output transformer of the valves Bi and B2 may therefore be represented by where S1 and S2 are the mutual conductances of the valves B1 and B2.

From this equation it may be seen that the signal A2 will'be completely suppressed if In the same manner the signalAimay be completely suppressed by making S1oc1=S21u2oc2. In'

1 It will be clear from the above consideration, that the invention enables a complete suppression of one of: two interfering signals.

A numerical example ofthe operation of the circuit is as follows.

Let the intensity'ratio of the two signals be 1/5. This ratio may be decreased to 1/20 by the detector D1 and to l/50 by the detector D2. Thus the voltage'in the output circuit of the detector D1 due to the strongersignal may be 1 volt and the voltage due to the weaker signal 0.05 volt, whereas in the output circuit of the detector D2 the voltage'due to the stronger signal may again be 1 volt, and the voltage due to the weaker signal 0.02 volt. Taking off 40% of theoutput volt- C1=20,u,u. F; C2= 1Q;L,u,F; 03:200 141 F; C4=10,u,u F; R1=R2=0.3 megohm.

WhatIclaim'is:" 7 r 1. A radio receiving circuit for the reception of said tunable circuit in parallel relation, both said detectors being arranged to simultaneously de- 'modulate the received signal voltage developed exclusively in said tunable circuit, a pair of low frequency amplifiers, a connection from each of the detector output circuits to its respective low frequency: amplifier, the output circuits of said amplifiers being connected in opposition- 2. A circuit arrangement comprising a tuned input circuit, a pair of diode rectifiers, the anode and cathode of each rectifier being connected across the tuned circuit through a condenser, a load resistor in the output circuit of each rectifier, the coupling condensers and the load resistors being so proportioned that the rectifiers have different time constants, an output amplifier stage having two tubes which have their output circuits connected in push-pull, and a variable tap connection from each loadresistor to one of the grids of the push-pull tubes.

CAREL JAN VAN LOON. 

